Silica sol as well as preparation method and application thereof

Abstract

The invention discloses silicon dioxide sol as well as a preparation method and application thereof. The surface of the silicon dioxide sol is rich in silicon hydroxyl; the silicon hydroxyl comprises two silicon atoms with different chemical shifts, namely a silicon atom Q2 connected with two hydroxyl groups and a silicon atom Q3 only connected with one hydroxyl group; the content ratio of Q2 to Q3 in the silicon dioxide sol is 0.05 to 0.20. The silicon dioxide sol provided by the invention is used as a carrier precursor and is uniformly mixed with active components in the acrylonitrile catalyst, the compounding effect is good, the active metal components of the catalyst are high in dispersity and not easy to agglomerate, and the catalyst is outstanding in catalytic activity and stability when being used in acrylonitrile synthesis reaction.

Description

Technical Field

[0001] This invention belongs to the field of industrial catalysts, specifically relating to a silica sol, its preparation method, and its application. Background Technology

[0002] Acrylonitrile is an important organic chemical raw material. Currently, its main production method involves reacting a catalyst made from active metals such as molybdenum and bismuth with propylene, ammonia, and oxygen in a fluidized bed. The metal component primarily bears the catalytic activity, while the silica support enhances the strength of the fluidized bed catalyst. Mixing silica sol with the active catalyst component and then spray-drying it can produce an acrylonitrile catalyst with uniform activity distribution, thus significantly improving the acrylonitrile yield. Researchers have conducted numerous studies on silica sol supports for acrylonitrile catalysts, finding that by optimizing stable silica sols with specific particle size and pH values, acrylonitrile catalysts with good catalytic activity can be prepared.

[0003] CN110562990 describes a method combining ion exchange and elemental silicon hydrolysis to prepare a silica sol with advantages such as narrow particle size distribution, low viscosity, and a gelation time exceeding one year at room temperature, which is used for the preparation of acrylonitrile catalysts. However, the silica sol preparation steps of this method are relatively cumbersome. CN111017935 discloses a silica sol for ceramic coatings, which modifies the silica sol by adding coupling agents such as titanate and aluminate. The coupling agents react with the hydroxyl groups on the silica sol surface, increasing the steric hindrance between the particle surfaces, thus improving the silica sol's gelation efficiency. The resulting silica sol particles are more stable, but this also introduces coupling agent components that are difficult to remove. In the preparation of silica sol, CN112499634 screened two reagents to modify the silica sol. The addition of sulfonic acid group side chains can effectively prevent the shrinkage and aggregation between silanol groups, and the addition of grafted cyclodextrin can improve the interfacial adsorption force on the silica surface. The two work synergistically to achieve the desired effect. The resulting silica sol particles have a particle size between 100-400 nanometers, a concentration of about 50%, a uniform particle size distribution, and good stability. However, the process is relatively complicated and the cost is high. Summary of the Invention

[0004] To address the problems in existing acrylonitrile catalysts, such as uneven mixing of the silica sol and active components, poor composite effect, low dispersion of the active metal component, and easy agglomeration, resulting in low catalytic activity and stability, this invention provides a silica sol, its preparation method, and its application. The silica sol provided by this invention, as a carrier precursor, mixes uniformly with the active components in the acrylonitrile catalyst, resulting in good composite effect, high dispersion of the active metal component, and resistance to agglomeration. When used in the synthesis of acrylonitrile, the catalyst exhibits outstanding catalytic activity and stability.

[0005] The first aspect of the present invention provides a silica sol, wherein the surface of the silica sol is rich in silanol groups;

[0006] The silanol groups contain two types of silicon atoms with different chemical shifts: silicon atoms connected to two hydroxyl groups (Q2) and silicon atoms connected to only one hydroxyl group (Q3); the content ratio of Q2 / Q3 in the silica sol is 0.05 to 0.20.

[0007] Furthermore, the Q2 / Q3 content ratio in the silica sol is preferably 0.10 to 0.15. The silanol content on the surface of the silica sol is controllable.

[0008] Furthermore, the sum of the contents of the two types of silicon atoms, Q2 and Q3, in the silica sol accounts for 20% to 40% of the total silicon atoms in the silica sol, preferably 23% to 35%.

[0009] Furthermore, the silica sol contains 18–45 wt% silica solids, preferably 20–40 wt%, and has a particle size of 10–25 nm, preferably 12–20 nm.

[0010] The silica sol described in this invention is a stable dispersion of silica nanoparticles in water.

[0011] A second aspect of the present invention provides a method for preparing the above-mentioned silica sol, comprising the following steps:

[0012] (1) A water glass solution is mixed with a cation exchange resin to carry out ion exchange and obtain an acidic active silica mother liquor;

[0013] (2) A portion of the acidic active silica mother liquor obtained in step (1) is aged to obtain silica sol seed crystals;

[0014] (3) Add the remaining acidic active silica mother liquor from step (1) to the silica sol seed crystals, and obtain crude silica sol product through alkaline catalytic reaction;

[0015] (4) The crude silica gel product is mixed with anion exchange resin for ion exchange, and the resulting dilute silica sol is concentrated by ultrafiltration to obtain silica sol.

[0016] Further, in step (1), the cation exchange resin is a strong acid cation exchange resin, preferably a 732 type strong acid styrene-based cation exchange resin.

[0017] Further, in step (1), the mass ratio of sodium silicate to cation exchange resin in the water glass solution is 1:5 to 1:50. The concentration of the water glass solution, calculated as Na₂SiO₃, is 5 to 25 wt%, preferably 10 to 20 wt%.

[0018] Furthermore, in step (1), the number of ion exchange cycles is 5 to 10, preferably 5 to 8;

[0019] Furthermore, in step (1), after the ion exchange, the pH of the acidic active silica mother liquor is 3.0 to 6.5.

[0020] Furthermore, in step (2), the mass of the acidic active silica mother liquor obtained in the partial step (1) accounts for 1% to 10% of the acidic active silica mother liquor obtained in step (1).

[0021] Further, in step (2), the maturation conditions are a maturation temperature of 75-95°C, preferably 80-90°C, a stirring speed of 150-300 rpm, preferably 180-280 rpm, and a maturation time of 8-24 h, preferably 10-18 h.

[0022] Further, in step (3), the alkaline catalyst in the alkaline catalytic reaction is at least one of ammonia, sodium hydroxide and urea, preferably ammonia with a concentration of 25-27 wt%.

[0023] Further, in step (3), the volume of the alkaline catalyst is 5-30% of the volume of the silica sol seed crystal, preferably 7-20%.

[0024] Further, in step (3), the temperature of the alkaline catalytic reaction is 90-100°C, the stirring speed is 250-500 rpm, preferably 320-400 rpm, and the time is 4-10 h, preferably 6-8 h.

[0025] Further, in step (4), the anion exchange resin is a strong base anion exchange resin, preferably a 717 type strong base styrene-based anion exchange resin.

[0026] Further, in step (4), the mass ratio of the crude silica gel product to the anion exchange resin is 0.8:1 to 1.5:1.

[0027] Furthermore, in step (4), the number of ion exchange cycles is 3 to 8 times, preferably 5 to 8 times.

[0028] Further, in step (4), after the ion exchange, solid impurities are removed by filtration, and the resulting dilute silica sol is concentrated.

[0029] Further, the ultrafiltration concentration in step (4) is carried out using an ultrafiltration membrane. Preferably, the ultrafiltration membrane is at least one of cellulose acetate membrane, polyethylene membrane, and polysulfone membrane, preferably a cellulose acetate membrane, and more preferably a cellulose acetate hollow fiber membrane.

[0030] Furthermore, the mass concentration of silica in the silica sol in step (4) is 18% to 45%.

[0031] A third aspect of the present invention also provides an acrylonitrile catalyst comprising a silica sol prepared according to any of the methods described in the first aspect or according to any of the methods described in the second aspect as a silica support component, and an active component.

[0032] Furthermore, the acrylonitrile catalyst contains 35% to 65% silica support by mass and 35% to 65% active component by mass.

[0033] Furthermore, the general formula of the active component is Mo. 12 Bi a B b C c D d O x Wherein, B is selected from at least one of Group VIII metal elements; C is selected from at least one of alkaline earth metal elements; D is selected from at least one of alkali metal elements; the value of a ranges from 0.05 to 10.0, the value of b ranges from 0.05 to 12.0, the value of c ranges from 0.02 to 8.0, and the value of d ranges from 0.02 to 2.0, where a, b, c, and d represent the molar content of Bi, B, C, and D relative to Mo, respectively; x is determined by the atomic ratio and valence state of the elements other than oxygen in the general formula of the active component.

[0034] The fourth aspect of the present invention provides a method for preparing the acrylonitrile catalyst described in the third aspect, comprising the following steps: mixing a metal active component precursor with silica sol to prepare a slurry, drying and calcining to obtain the acrylonitrile catalyst.

[0035] Furthermore, the metal in the metal active component precursor includes Mo as well as Bi, B, C, and D.

[0036] Furthermore, the precursor of the metal active component is a water-soluble salt containing an active metal element, such as a Mo source being a Mo-containing oxyacid ammonium salt, preferably (NH4)6Mo7O. 24 Or its hydrate; the remaining Bi, B, C and D sources are preferably one or more of halides, alkoxides, nitrates or acetates, preferably nitrates, such as bismuth nitrate, ferric nitrate, nickel nitrate, magnesium nitrate, etc.

[0037] Furthermore, the reaction time is 10–40 min, preferably 20–30 min, the temperature is 100–160 °C, preferably 120–150 °C, and the stirring speed is 150–250 rpm.

[0038] Furthermore, the drying is preferably spray drying, with air as the heat source, a drying temperature of 300–450°C, preferably 320–400°C, a drying time of 0.2–4.0 h, preferably 0.5–1.5 h, and an average diameter of spray droplets of 30–150 μm, preferably 30–120 μm.

[0039] Furthermore, the calcination conditions are as follows: calcination is carried out in an oxygen-containing atmosphere, such as in an air atmosphere; the calcination temperature is 400–700°C, preferably 450–640°C, and the calcination time is 3–8 hours, preferably 4–6 hours.

[0040] The fifth aspect of the present invention also provides the application of the acrylonitrile catalyst described in the third aspect above or the acrylonitrile catalyst prepared in the fourth aspect in the ammoxidation of propylene to acrylonitrile reaction.

[0041] Furthermore, in the aforementioned application, propylene undergoes an ammoxidation reaction to produce acrylonitrile in the presence of ammonia and oxygen in the presence of an acrylonitrile catalyst.

[0042] Further, the reaction conditions include: a molar ratio of propylene / ammonia / air (based on oxygen content) of 1:1.1–1.3:9.5–10.0, a reaction temperature of 420–440°C, a reaction pressure (gauge pressure) of 0.06–0.12 MPa, and a catalyst loading (volume) of 0.08–0.12 h⁻¹. -1 .

[0043] Compared with the prior art, the present invention has the following advantages:

[0044] In actual production, the inventors discovered that, in addition to factors such as particle size and pH value, the surface properties of the carrier silica sol also have a significant impact on the performance of the final catalyst.

[0045] Further research by the inventors revealed that the silica sol provided by this invention has the following advantages: (1) It has specific silanol groups on its surface, which can coordinate with active metals to form a stable slurry with uniformly dispersed active components during catalyst preparation; (2) It has good stability and uniform particle distribution; (3) When used as a carrier, the active components of the catalyst are uniformly dispersed and have less agglomeration. In the reaction of propylene ammoxidation to acrylonitrile, it not only has good activity but also good long-term stability. Attached Figure Description

[0046] Figure 1 This is a transmission electron microscope (TEM) image of the silica sol in Example 1;

[0047] Figure 2 The silica sol in Example 1 29 Si NMR spectrum;

[0048] Figure 3 This is a TEM image of the acrylonitrile catalyst prepared from silica sol in Example 1;

[0049] Figure 4 TEM image of the acrylonitrile catalyst prepared from silica sol in Comparative Example 1;

[0050] Figure 5 This is a TEM image of the silica sol in Comparative Example 1. Detailed Implementation

[0051] The technical solution of the present invention will be further illustrated below through embodiments. These embodiments are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0052] In the context of this specification, including in the following examples and comparative examples, an inductively coupled plasma atomic emission spectrometer (ICP) model Varian 725-ES was used to determine the elemental content, in molars, by dissolving the analytical sample in hydrofluoric acid.

[0053] In the context of this specification, including in the following examples and comparative examples, the content of different types of silicon atoms in the silica sol is determined by... 29 Measured using a Bruker AvanceⅢ / WB-400 NMR spectrometer, the peak with a chemical shift of approximately 92 ppm corresponds to silicon atoms with two hydroxyl groups (Q2), while the peak with a chemical shift of approximately 101 ppm corresponds to silicon atoms with only one hydroxyl group (Q3). The ratio of the peak area corresponding to Q2 to the peak area corresponding to Q3 represents the content ratio of the two types of silicon atoms, Q2 / Q3. The percentage of the sum of the peak areas corresponding to Q2 and Q3 to the total peak area of ​​silicon atoms represents the content of Q2 and Q3 silicon atoms in the silica sol.

[0054] In the context of this specification, the transmission electron microscope (TEM) used for the silica sol is an S-4800II field emission scanning electron microscope. The particle size in the silica sol was calculated by statistically analyzing the sizes of approximately 30 silicon particles from the TEM images and then averaging them.

[0055] In this invention, gas chromatography is used for online analysis of the product gas. The conversion rate of propylene, the selectivity of acrylonitrile, and the single-pass yield are used as indicators to evaluate the catalyst performance. The definitions of these two parameters are as follows:

[0056] Propylene conversion rate (%) = (moles of propylene reacted / moles of propylene fed) × 100%,

[0057] Acrylonitrile selectivity (%) = (moles of acrylonitrile produced / moles of propylene reacted) × 100%.

[0058] Acrylonitrile single-pass yield (%) = (moles of acrylonitrile produced / moles of propylene fed) × 100%.

[0059] In this invention, the catalyst composition is based on the amount of raw materials fed. The catalyst load during the reaction refers to the amount of gas processed per unit volume of catalyst per unit time, which is essentially the volume hourly space velocity (VHSV).

[0060] Example 1

[0061] (1) Preparation of silica sol

[0062] 268.0 g of Na₂SiO₃·9H₂O was dissolved in 500 mL of deionized water to prepare a Na₂SiO₃ solution. 3000 g of type 732 strong acid styrene-based cation exchange resin was dispersed in 2 L of deionized water, and the Na₂SiO₃ solution was added. The mixture was stirred at 250 rpm for 1 h at 40 °C, filtered, and the mixture was circulated 8 times to obtain an acidic active silica mother liquor (approximately 2800 mL) with pH = 4.2. 60 mL of this acidic active silica mother liquor was weighed and heated at 220 rpm at 90 °C for aging, and stirred for 12 h to obtain silica sol seed crystals (approximately 50 mL). Then, the remaining acidic active silica mother liquor (approximately 2740 mL) and 10 mL of 27% ammonia solution were slowly added to the silica sol seed crystals. The mixture was heated at 95 °C for 6 h with a stirring rate of 380 rpm to obtain crude silica sol. Then, approximately 2800 g of the crude silica sol was mixed with 2500 g of type 717 anion exchange resin for 1 hour and filtered for separation, repeating the process 4 times. Finally, the mixture was filtered to remove a small amount of solid impurities, and then subjected to ultrafiltration through a cellulose acetate hollow fiber membrane to obtain a silica sol with a concentration of 32%. Its TEM image is shown below. Figure 1 As shown, the particle size in the silica sol is 12 nm. 29 Si NMR spectrum as shown Figure 2 As shown, the calculated Q2 content is 2.84%, the Q3 content is 23.56%, and the ratio of the two is 0.12. The two types of silicon atoms (Q2+Q3) that connect to hydroxyl groups account for 26.4% of the total silicon atoms in the silica sol.

[0063] (2) Preparation of acrylonitrile catalyst

[0064] 8.6 g Bi(NO3)3·5H2O, 28.6 g Ni(NO3)2·6H2O, 15.9 g Fe(NO3)3·9H2O, 6.6 g Mg(NO3)2·6H2O, and 0.26 g KOH were heated and dissolved, and then thoroughly mixed. 150 g of the silica sol prepared in step (1) was added to the mixture, and thoroughly mixed to obtain solution I; 47.9 g (NH4)6Mo7O 24 • 4H₂O was dissolved in 100 mL of hot water to obtain solution II; solution II was added to solution I, and the mixture was stirred at 220 rpm for 20 min at 150 °C to obtain a mixed slurry. The mixed slurry was spray-dried with an average droplet diameter of 50 μm at 350 °C for 0.5 h to obtain powder particles. The particles were calcined at 600 °C for 4 h in air to obtain an acrylonitrile catalyst. The mass content of the active component was 55%. The general formula of the active component is Mo. 12 Bi 0.8 B 6.1 C 1.1 D 0.2 O x Its TEM image is as follows: Figure 3 As shown, the overall active components are dispersed very evenly, with no obvious clumping or agglomeration.

[0065] (3) Evaluation of catalyst activity

[0066] In a φ38 mm fluidized bed reactor, the reaction temperature was 420℃, the reaction pressure was 0.085 MPa, and the catalyst propylene loading (WWH) was 0.12 hours. -1 Raw material ratio (molar): C3 ＝ / NH3 / air = 1 / 1.26 / 9.6. Propylene conversion rate is 99.48%, acrylonitrile selectivity is 84.47%, and acrylonitrile single-pass yield is 84.03%.

[0067] Example 2

[0068] (1) Preparation of silica sol

[0069] 268.0 g of Na₂SiO₃·9H₂O was dissolved in 500 mL of deionized water to prepare a Na₂SiO₃ solution. 5000 g of type 732 strong acid styrene-based cation exchange resin was dispersed in 3 L of deionized water, and the Na₂SiO₃ solution was added. The mixture was stirred at 250 rpm for 1 h at 40 °C, filtered, and the mixture was circulated 8 times to obtain an acidic active silicic acid mother liquor (approximately 3800 mL) with pH = 3.8. 81 mL of this acidic active silicic acid mother liquor was weighed and heated at 220 rpm at 90 °C for aging, and stirred for 12 h to obtain silica sol seed crystals (70 mL). Then, the remaining acidic active silicic acid mother liquor (approximately 3720 mL) and 10 mL of 27% ammonia solution were slowly added to the silica sol seed crystals. The mixture was heated at 95 °C for 6 h with a stirring rate of 380 rpm to obtain crude silica sol. The crude silica sol was then mixed with 4200 g of 717 type strong base styrene-based anion exchange resin for 1 hour and filtered for separation, repeating the process 4 times. Finally, the small amount of solid impurities was removed by filtration, and the mixture was then subjected to ultrafiltration through a cellulose acetate hollow fiber membrane to obtain a silica sol with a concentration of 34%. Calculations showed that its Q2 content was 2.43%, and its Q3 content was 22.86%, with a ratio of 0.11. The two types of silicon atoms (Q2+Q3) linking hydroxyl groups accounted for 25.29% of the total silicon atoms in the silica sol.

[0070] (2) The preparation steps of the acrylonitrile catalyst are the same as in Example 1.

[0071] (3) Evaluation of catalyst activity

[0072] In a φ38 mm fluidized bed reactor, the reaction temperature was 420℃, the reaction pressure was 0.085 MPa, and the catalyst propylene loading (WWH) was 0.12 hours. -1 Raw material ratio (molar): C3 ＝ / NH3 / air = 1 / 1.26 / 9.6. Propylene conversion rate is 99.35%, acrylonitrile selectivity is 84.42%, and acrylonitrile single-pass yield is 83.87%.

[0073] Example 3

[0074] (1) Preparation of silica sol

[0075] 268.0 g of Na₂SiO₃·9H₂O was dissolved in 500 mL of deionized water to prepare a Na₂SiO₃ solution. 3000 g of type 732 strong acid styrene-based cation exchange resin was dispersed in 2 L of deionized water, and the Na₂SiO₃ solution was added. The mixture was stirred at 250 rpm for 1 h at 40 °C, filtered, and the mixture was circulated 8 times to obtain an acidic active silicic acid mother liquor (approximately 2800 mL) with pH = 4.4. 150 mL of this acidic active silicic acid mother liquor was weighed and heated at 220 rpm at 90 °C for aging, and stirred for 12 h to obtain silica sol seed crystals (130 mL). Then, the remaining acidic active silicic acid mother liquor (approximately 2650 mL) and 10 mL of 27% ammonia solution were slowly added to the silica sol seed crystals. The mixture was heated at 95 °C for 6 h with a stirring rate of 380 rpm to obtain crude silica sol. The crude silica sol was then mixed with 2500 g of 717 type strong base styrene-based anion exchange resin for 1 hour and filtered for separation, repeating the process 4 times. Finally, the small amount of solid impurities was removed by filtration, and the mixture was then subjected to ultrafiltration through a cellulose acetate hollow fiber membrane to obtain a silica sol with a concentration of 32%. The calculated Q2 content was 2.60%, the Q3 content was 25.56%, and the ratio was 0.10. The two types of silicon atoms (Q2+Q3) linking hydroxyl groups accounted for 28.16% of the total silicon atoms in the silica sol.

[0076] (2) The preparation steps of the acrylonitrile catalyst are the same as in Example 1.

[0077] (3) Evaluation of catalyst activity

[0078] In a φ38 mm fluidized bed reactor, the reaction temperature was 420℃, the reaction pressure was 0.085 MPa, and the catalyst propylene loading (WWH) was 0.12 hours. -1 Raw material ratio (molar): C3 ＝ / NH3 / air = 1 / 1.26 / 9.6. Propylene conversion rate is 99.34%, acrylonitrile selectivity is 84.49%, and acrylonitrile single-pass yield is 83.93%.

[0079] Example 4

[0080] (1) Preparation of silica sol

[0081] 268.0 g of Na₂SiO₃·9H₂O was dissolved in 500 mL of deionized water to prepare a Na₂SiO₃ solution. 3000 g of 732 type strong acid styrene-based cation exchange resin was dispersed in 2 L of deionized water, and the Na₂SiO₃ solution was added. The mixture was stirred at 250 rpm for 1 h at 40 °C, filtered, and the mixture was circulated 8 times to obtain an acidic active silicic acid mother liquor (approximately 2800 mL) with pH = 4.5. 60 mL of this acidic active silicic acid mother liquor was weighed and heated at 220 rpm at 80 °C for aging, and stirred for 18 h to obtain silica sol seed crystals (50 mL). Then, the remaining acidic active silicic acid mother liquor (approximately 2740 mL) and 10 mL of 27% urea solution were slowly added to the silica sol seed crystals. The mixture was heated at 90 °C for 6 h with a stirring rate of 380 rpm to obtain crude silica sol. The crude silica sol was then mixed with 2500 g of 717 type strong base styrene-based anion exchange resin for 1 hour and filtered for separation, repeating the process 4 times. Finally, the small amount of solid impurities was removed by filtration, and the mixture was ultrafiltered through a cellulose acetate hollow fiber membrane to obtain a silica sol with a concentration of 31%. The calculated Q2 content was 2.25%, the Q3 content was 20.76%, and the ratio of the two was 0.11. The two types of silicon atoms (Q2+Q3) that are linked to hydroxyl groups accounted for 23.01% of the total silicon atoms in the silica sol.

[0082] (2) The preparation steps of the acrylonitrile catalyst are the same as in Example 1.

[0083] (3) Evaluation of catalyst activity

[0084] In a φ38 mm fluidized bed reactor, the reaction temperature was 420℃, the reaction pressure was 0.085 MPa, and the catalyst propylene loading (WWH) was 0.12 hours. -1 Raw material ratio (molar): C3 ＝ / NH3 / air = 1 / 1.26 / 9.6. Propylene conversion rate is 99.17%, acrylonitrile selectivity is 84.20%, and acrylonitrile single-pass yield is 83.50%.

[0085] Comparative Example 1

[0086] (1) Preparation of silica sol

[0087] 268.0 g of Na₂SiO₃·9H₂O was dissolved in 500 mL of deionized water to prepare a Na₂SiO₃ solution. 3000 g of D113 macroporous weakly acidic cation exchange resin was dispersed in 2 L of deionized water, and the Na₂SiO₃ solution was added. The mixture was stirred at 250 rpm for 1 h at 40 °C, filtered, and the mixture was circulated 8 times to obtain an acidic active silicic acid mother liquor (approximately 2800 mL) with pH = 5.4. 60 mL of this acidic active silicic acid mother liquor was weighed and heated at 220 rpm at 90 °C for aging, and stirred for 12 h to obtain 50 mL of silica sol seed crystals. Then, the remaining acidic active silicic acid mother liquor (approximately 2740 mL) and 10 mL of 27% ammonia solution were slowly added to the silica sol seed crystals. The mixture was heated at 95 °C for 6 h with a stirring rate of 380 rpm to obtain crude silica sol. The crude silica sol was then mixed with 2500 g of D301 macroporous weakly basic anion exchange resin for 1 hour and filtered for separation, repeating the process 4 times. Finally, the small amount of solid impurities was removed by filtration, and the mixture was then subjected to ultrafiltration through a cellulose acetate hollow fiber membrane to obtain a silica sol with a concentration of 30%. The Q2 content was 0.25%, the Q3 content was 14.37%, and the ratio of the two was 0.017. The two types of silicon atoms (Q2+Q3) that are linked to hydroxyl groups accounted for 14.62% of the total silicon atoms in the silica sol.

[0088] (2) The preparation steps of the acrylonitrile catalyst are the same as in Example 1.

[0089] (3) Evaluation of catalyst activity

[0090] In a φ38 mm fluidized bed reactor, the reaction temperature was 420℃, the reaction pressure was 0.085 MPa, and the catalyst propylene loading (WWH) was 0.12 hours. -1 Raw material ratio (molar): C3 ＝ / NH3 / air = 1 / 1.26 / 9.6. Propylene conversion rate is 95.20%, acrylonitrile selectivity is 80.35%, and acrylonitrile single-pass yield is 76.49%.

[0091] Comparative Example 2

[0092] (1) Preparation of silica sol

[0093] 268.0 g of Na₂SiO₃·9H₂O was dissolved in 500 mL of deionized water to prepare a Na₂SiO₃ solution. 1200 g of type 732 cation exchange resin was dispersed in 480 mL of deionized water, and the Na₂SiO₃ solution was added to it. The mixture was stirred at 250 rpm for 1 h at 40 °C, filtered, and the mixture was circulated 8 times to obtain an acidic active silicic acid mother liquor (approximately 1200 mL) with pH = 5.8. 25 mL of the acidic active silicic acid mother liquor was weighed and heated at 220 rpm at 90 °C for aging, and stirred for 12 h to obtain 20 mL of silica sol seed crystals. Then, the remaining acidic active silicic acid mother liquor (approximately 1175 mL) and 10 mL of 27% ammonia solution were slowly added to the silica sol seed crystals. The mixture was heated at 95 °C for 6 h with a stirring rate of 380 rpm to obtain crude silica sol. The crude silica sol was then mixed with 2500 g of 717 type anion exchange resin for 1 hour and filtered for separation, repeating the process 4 times. Finally, the small amount of solid impurities was removed by filtration, and the mixture was ultrafiltered through a cellulose acetate hollow fiber membrane to obtain a silica sol with a concentration of 28%. The calculated Q2 content was 0.42%, the Q3 content was 16.15%, and the ratio of the two was 0.026. The two types of silicon atoms (Q2+Q3) that are linked to hydroxyl groups accounted for 16.57% of the total silicon atoms in the silica sol.

[0094] (2) The preparation steps of the acrylonitrile catalyst are the same as in Example 1.

[0095] (3) Evaluation of catalyst activity

[0096] In a φ38 mm fluidized bed reactor, the reaction temperature was 420℃, the reaction pressure was 0.085 MPa, and the catalyst propylene loading (WWH) was 0.12 hours. -1 Raw material ratio (molar): C3 ＝ / NH3 / air = 1 / 1.26 / 9.6. Propylene conversion rate is 94.21%, acrylonitrile selectivity is 79.42%, and acrylonitrile single-pass yield is 74.82%.

[0097] Comparative Example 3

[0098] (1) Preparation of silica sol

[0099] 268.0 g of Na₂SiO₃·9H₂O was dissolved in 500 mL of deionized water to prepare a Na₂SiO₃ solution. 3000 g of type 732 cation exchange resin was dispersed in 2 L of deionized water, and the Na₂SiO₃ solution was added. The mixture was stirred at 250 rpm for 1 h at 40 °C, filtered, and the mixture was circulated 8 times to obtain an acidic active silicic acid mother liquor (approximately 2800 mL) with pH = 5.2. 300 mL of this acidic active silicic acid mother liquor was weighed and heated at 220 rpm at 100 °C for aging, and stirred for 12 h to obtain 250 mL of silica sol seed crystals. Then, the remaining acidic active silicic acid mother liquor (approximately 2500 mL) and 10 mL of 27% ammonia solution were slowly added to the silica sol seed crystals. The mixture was heated at 90 °C for 6 h with a stirring rate of 380 rpm to obtain crude silica sol. The crude silica sol was then mixed with 2500 g of 717 type anion exchange resin for 1 hour and filtered for separation, repeating the process 4 times. Finally, the small amount of solid impurities was removed by filtration, and the mixture was ultrafiltered through a cellulose acetate hollow fiber membrane to obtain a silica sol with a concentration of 30%. The calculated Q2 content was 1.02%, the Q3 content was 18.23%, and the ratio of the two was 0.056. The two types of silicon atoms (Q2+Q3) that are linked to hydroxyl groups accounted for 19.25% of the total silicon atoms in the silica sol.

[0100] (2) The preparation steps of the acrylonitrile catalyst are the same as in Example 1.

[0101] (3) Evaluation of catalyst activity

[0102] In a φ38 mm fluidized bed reactor, the reaction temperature was 420℃, the reaction pressure was 0.085 MPa, and the catalyst propylene loading (WWH) was 0.12 hours. -1 Raw material ratio (molar): C3 ＝ / NH3 / air = 1 / 1.26 / 9.6. Propylene conversion rate is 93.10%, acrylonitrile selectivity is 77.65%, and acrylonitrile single-pass yield is 72.29%.

[0103] Comparative Example 4

[0104] (1) Preparation of silica sol

[0105] 268.0 g of Na₂SiO₃·9H₂O was dissolved in 500 mL of deionized water to prepare a Na₂SiO₃ solution. 3000 g of type 732 cation exchange resin was dispersed in 2 L of deionized water, and the Na₂SiO₃ solution was added. The mixture was stirred at 250 rpm for 1 h at 40 °C, filtered, and the mixture was circulated 8 times to obtain an acidic active silicic acid mother liquor (approximately 2800 mL) with pH = 4.2. 60 mL of this acidic active silicic acid mother liquor was weighed and heated at 220 rpm at 90 °C for aging, and stirred for 12 h to obtain 50 mL of silica sol seed crystals. Then, the remaining acidic active silicic acid mother liquor (approximately 2740 mL) and 10 mL of 27% ammonia solution were slowly added to the silica sol seed crystals. The mixture was heated at 95 °C for 6 h with a stirring rate of 380 rpm to obtain crude silica sol. The crude silica sol was then mixed with 2500 g of 717 type anion exchange resin for 1 hour and filtered for separation, repeating the process 4 times. Finally, the small amount of solid impurities was removed by filtration, and the mixture was concentrated by stirring and evaporation at 110℃ for 8 hours to obtain a silica sol with a concentration of 26%. The calculated Q2 content was 0.16%, the Q3 content was 10.55%, and the ratio of the two was 0.015. The two types of silicon atoms (Q2+Q3) that are linked to hydroxyl groups accounted for 10.71% of the total silicon atoms in the silica sol.

[0106] (2) The preparation steps of the acrylonitrile catalyst are the same as in Example 1.

[0107] (3) Evaluation of catalyst activity

[0108] In a φ38 mm fluidized bed reactor, the reaction temperature was 420℃, the reaction pressure was 0.085 MPa, and the catalyst propylene loading (WWH) was 0.12 hours. -1 Raw material ratio (molar): C3 ＝ / NH3 / air = 1 / 1.26 / 9.6. Propylene conversion rate is 90.86%, acrylonitrile selectivity is 75.63%, and acrylonitrile single-pass yield is 68.72%.

Claims

1. A silica sol, characterized in that, The surface of the silica sol is rich in silanol groups; The silanol groups contain two types of silicon atoms with different chemical shifts: silicon atoms Q2 that are connected to two hydroxyl groups and silicon atoms Q3 that are connected to only one hydroxyl group; the content ratio of Q2 / Q3 in the silica sol is 0.05 to 0.

20.

2. The silica sol according to claim 1, characterized in that, The sum of the contents of the two types of silicon atoms, Q2 and Q3, in the silica sol accounts for 20% to 40% of the total silicon atoms in the silica sol.

3. The silica sol according to claim 1, characterized in that, The silica sol contains 18–45 wt% silica solids, preferably 20–40 wt%, and has a particle size of 10–25 nm, preferably 12–20 nm.

4. A method for preparing silica sol according to any one of claims 1-3, comprising the following steps: (1) A water glass solution is mixed with a cation exchange resin to carry out ion exchange and obtain an acidic active silica mother liquor; (2) A portion of the acidic active silica mother liquor obtained in step (1) is aged to obtain silica sol seed crystals; (3) Add the remaining acidic active silica mother liquor from step (1) to the silica sol seed crystals, and obtain crude silica sol product through alkaline catalytic reaction; (4) The crude silica gel product is mixed with anion exchange resin for ion exchange, and the resulting dilute silica sol is concentrated by ultrafiltration to obtain silica sol.

5. The preparation method according to claim 4, characterized in that, In step (1), the cation exchange resin is a strong acid cation exchange resin, preferably a 732 type strong acid styrene-based cation exchange resin; And / or, in step (1), the mass ratio of sodium silicate to cation exchange resin in the water glass solution is 1:5 to 1:

50.

6. The preparation method according to claim 4, characterized in that, In step (1), the ion exchange cycle is 5 to 10 times; And / or, in step (1), after the ion exchange, the pH value of the acidic active silica mother liquor is 3.0 to 6.

5.

7. The preparation method according to claim 4, characterized in that, In step (2), the mass of the acidic active silica mother liquor obtained in the partial step (1) accounts for 1% to 10% of the acidic active silica mother liquor obtained in step (1).

8. The preparation method according to claim 4, characterized in that, In step (2), the maturation conditions are a maturation temperature of 75-95℃, a stirring speed of 150-300rpm, and a maturation time of 8-24h.

9. The preparation method according to claim 4, characterized in that, In step (3), the alkaline catalyst in the alkaline catalytic reaction is at least one of ammonia, sodium hydroxide and urea, preferably ammonia with a concentration of 25-27 wt%. And / or, in step (3), the volume of the alkaline catalyst is 5 to 30% of the volume of the silica sol seed crystal; And / or, in step (3), the temperature of the alkaline catalytic reaction is 90-100°C, the stirring speed is 250-500 rpm, and the time is 4-10 h.

10. The preparation method according to claim 4, characterized in that, In step (4), the anion exchange resin is a strong base anion exchange resin, preferably a 717 type strong base styrene-based anion exchange resin. And / or, in step (4), the mass ratio of the crude silica gel product to the anion exchange resin is 0.8:1 to 1.5:1; And / or, in step (4), the number of ion exchange cycles is 3 to 8.

11. The preparation method according to claim 4, characterized in that, The ultrafiltration concentration in step (4) is carried out by ultrafiltration membrane concentration. Preferably, the ultrafiltration membrane is at least one of cellulose acetate membrane, polyethylene membrane, and polysulfone membrane, and more preferably cellulose acetate membrane. And / or, the mass concentration of silica in the silica sol in step (4) is 18% to 45%.

12. An acrylonitrile catalyst comprising a silica sol as described in any one of claims 1-3 or a silica sol prepared according to any one of claims 4-11 as a silica support precursor, and an active component.

13. The acrylonitrile catalyst according to claim 12, characterized in that, The acrylonitrile catalyst contains 35% to 65% silica support by mass and 35% to 65% active component by mass.

14. The acrylonitrile catalyst according to claim 12, characterized in that, The general formula of the active component is Mo. 12 Bi a B b C c D d O x Wherein, B is selected from at least one of Group VIII metal elements; C is selected from at least one of alkaline earth metal elements; D is selected from at least one of alkali metal elements; the value of a ranges from 0.05 to 10.0, the value of b ranges from 0.05 to 12.0, the value of c ranges from 0.02 to 8.0, and the value of d ranges from 0.02 to 2.0, where a, b, c, and d represent the molar content of Bi, B, C, and D relative to Mo, respectively; x is determined by the atomic ratio and valence state of the elements other than oxygen in the general formula of the active component.

15. The use of the acrylonitrile catalyst according to any one of claims 12-14 in the ammoxidation of propylene to acrylonitrile; Optionally, the reaction conditions include: The molar ratio of propylene / ammonia / air (based on oxygen content) is 1: 1.1~1.3: 9.5~10.0, reaction temperature 420~440℃, reaction pressure (gauge pressure) 0.06~0.12MPa, catalyst loading 0.08~0.12h. -1 .

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